1. Field of the Invention
The present invention relates to an acoustic detector for detecting the presence of hard foreign objects in a material being transported along a feed path by a feed mechanism. More particularly, the present invention relates to an acoustic detector having inhibit means associated therewith for inhibiting any output signal from the detector during periods of time when the feed mechanism is stopped or is moving at less than its normal operating speed. The invention is particularly suited for use with agricultural harvesting equipment but finds use in material feed mechanisms of general utility.
2. Description of Prior Art
The problem of hard foreign materials in crop material being processed by agricultural harvesting equipment moving across farm fields is well known and much time and money has been expended in efforts to solve the problem. Magnetic detection systems such as that disclosed in U.S. Pat. No. 3,959,953 to Garrott have been developed for the detection of ferrous metal objects in crop material. However, magnetic detection systems are not capable of detecting non-ferrous hard objects such as stones.
U.S. Pat. No. 4,353,199 to Chow et al. discloses an acoustic system for detecting the presence of stones and other hard foreign objects in crop material being fed through a forage harvester. In this system a piezoelectric acoustic sensor is mounted on the interior surface of a rotatable crop feed roll so as to sense vibrations induced in the feed roll by hard objects striking its exterior surface. However, this system requires that the feed roll be shock mounted so as to isolate the feed roll (and thus the sensor) from machine generated vibrations. Furthermore, since the sensor rotates with the feed roll in which it is mounted, a rotary coupling or slip ring must be provided in order to transmit signals generated by the sensor to circuitry, external of the feed roll, which controls the stopping of the feed mechanism of the harvester.
The concurrently filed application of Carl E. Bohman et al., Ser. No. 07/603,606 discloses and claims an acoustic detector system for detecting stones and other hard objects in a material being transported along a feed path of a forage harvester by a feed mechanism, the acoustic detector system including a microphone disposed in a fixed position inside a feed roll and producing output signals in response to air-borne sound waves induced inside the feed roll by impacts of hard foreign objects on the feed mechanism. The microphone responds to the sound waves by producing an output signal for stopping the feed mechanism.
Acoustic hard object detection systems, whether responsive to air-borne sound waves as in the Bohman et al. application or mechanically transmitted vibrations as in the Chow et al. patent, are sensitive to sounds and vibrations produced as drive power is initially applied to the feed mechanism. Also, the impact of tools on the harvester during repair and adjustment periods tend to trigger the detector system thereby stopping the feed mechanism. To overcome these problems, it has been customary to reduce the sensitivity of the acoustic detector system. However, this reduced sensitivity reduces the ability of the detector system to detect small stones and other hard objects during normal operation.